1. Field of the Invention
The present invention relates to a semiconductor device and a manufacturing method thereof.
2. Description of Related Art
With the rapid advent of portable devices such as mobile phones and IC cards in recent years, and the resulting need for thinner, lighter, more compact resin-sealed semiconductor devices to be mounted in those devices, a great number of means to fulfill such need have been proposed in the prior art. In one such instance, a method for mounting a resin-sealed semiconductor device 101 at an external substrate 102 such as a printed circuit board, as illustrated in FIG. 31, is proposed.
In the resin-sealed semiconductor device 101 that achieves this mounting state, a mold portion 104 constituted by sealing a semiconductor chip is provided at a front surface of a substrate 103 constituted of an epoxy resin, ceramic or the like and solder balls 105 to function as solder bumps are bonded in advance at specific connecting areas at a rear surface of the substrate 103. Then, the semiconductor device 101 in this state is placed on the external substrate 102 and the entire assembly is placed in an atmosphere at a temperature ranging approximately from 220 centigrade to 240 centigrade. Thus, the solder balls 105 are at least partially melted so that the semiconductor device 101 can be mounted at the external substrate 102. Through this mounting method, electrical characteristics with a low capacity and low inductance are achieved.
To describe the structure of the resin-sealed semiconductor device 101 that achieves this mounting state in more detail, in reference to FIG. 32, a metal pattern 106 formed at a rear surface of the substrate 103 is covered with an insulating film 107 constituted of, for instance, asolder resist, except at a specific connecting area 106a, the connecting area 106a is set so that it lies flush with a front surface of the insulating film 107 and a surface of the connecting area 106a is flat. A solder ball 105 is bonded to the connecting area 106a.
When mounting the semiconductor device 101 at the external substrate 102, the solder ball 105 is aligned at a specific electrode portion 109 formed between insulating layers 108 and 108 and then the entire assembly is placed in a specific heated atmosphere in this state.
However, the following problem manifests with the semiconductor device 101 in the prior art structured as described above in a reliability test which is performed after it is mounted at the external substrate 102. Namely, during the reliability test in which the semiconductor device is exposed to an atmosphere at room temperature or within the range of -65 centigrade to 150 centigrade, an electrical disconnection sometimes occurs at the solder balls 105. This is considered to be caused by the difference between the coefficients of thermal expansion of the semiconductor device 101 constituted of the substrate 103 and the mold portion 104 and of the external substrate 102 at which the semiconductor device 101 is mounted, which causes the semiconductor device 101 to be stretched and to contract, as illustrated in FIG. 33, causing cracks 110 and 111 to be formed at a solder ball 105, which, in turn, leads to degradation of the electrical characteristics and eventually to disconnection as these cracks 110 and 111 grow to link with each other.
In an examination of the positions at which the cracks 110 and 111 are formed, their patterns and their directions, conducted by the inventors of the present invention, cracks were found to form near the metal pattern 106 to extend in parallel to the metal pattern 106 in most instances. The inventors of the present invention conducted a similar experiment after essentially modifying the shape of the metal pattern connecting area which is bonded with the solder ball, and a great improvement was observed.